This invention relates to electronic device fabrication processes. More specifically, the invention relates to chemical vapor deposition processes for forming dielectric layers in high aspect ratio, narrow width recessed features.
It is often necessary in semiconductor processing to fill a trench or a high aspect ratio gaps with insulating material. This is the case for shallow trench isolation (STI) layers, inter-metal dielectric (IMD) layers, inter-layer dielectric (ILD) layers, passivation layers, etc. As device geometries shrink and thermal budgets are reduced, void-free filling or uniform lining of high aspect ratio spaces (e.g., AR>6:1) becomes increasingly difficult due to limitations of existing deposition processes.
One approach to gap fill is high-density plasma chemical vapor deposition (HDP CVD). HDP CVD is a directional (bottom-up) CVD process that is used for high aspect ratio gap-fill. The method deposits more material at the bottom of a high aspect ratio structure than on its sidewalls, which is accomplished by directing charged dielectric precursor species downward, to the bottom of the gap, while simultaneously removing deposited material from the trench top through sputtering using the biased RF power applied to the substrate. Nevertheless, HDP CVD gap fill results in the formation of cusps, also known as overhangs, at the entry region of the gap to be filled. This results from the non-directional deposition reactions of species in the plasma reactor and from sputtering and re-deposition processes. The directional aspect of the deposition process produces some high momentum charged species that sputter away bottom fill. The sputtered material tends to redeposit on the sidewalls. As a result, the entry region of a high aspect ratio structure may close before bottom-up fill has been completed, leaving voids or weak spots within the structure. This phenomenon, known as “pinch-off,” is exacerbated in narrow features. Limitations due to overhang formation become even more severe as the width of the gap to be filled decreases and the aspect ratio increases.